Disasters of various kinds have necessitated creating of warning systems that deliver initial warnings directly to mobile telephones. Such warnings are desired to better reach people than earlier used radio broadcasts over traditional FM or AM radios and warning signals delivered by loudspeakers. There are also many locations that lack such loudspeakers that could be heard.
For a warning system to work properly, people must be reached and the message be conveyed as soon as possible. Mobile telephones operate in radio networks where the network resource is a scarce resource that does not suffice for simultaneous use of everyone in a point to point manner. Moreover, when a warning of a large disaster is announced, it is typical that people start to place phone calls and send short messages which in turn adds congestion in the radio networks.